The present invention refers to a method for an integrated treatment of cellulose pulp, including providing said cellulose pulp, providing a determined quantity of white liquor including alkali and sulphur components and providing an oxygen-containing gas. More precisely, the invention refers to the production of pulp and is especially directed to the so called oxygen delignification stage.
In sulphate pulp production intended for the manufacture of bleached/white cellulose pulp for white products, such as printing paper, white cardboard, hygienic products, the aim of the process is to remove as much lignin as possible. The largest quantity is dissolved already during the pulping where the wood chips are pulped in white liquor. In the white liquor both hydroxide ions (=alkali), for instance from NaOH, and hydrogen sulphide ions and their derivates are active components. The white liquor is manufactured in situ and consumed liquor is recovered after having passed the chemical recovering cycle.
Thereafter, the pulped cellulose pulp is most frequently supplied to an oxygen delignification stage, where alkali and oxygen are added. Today, this conventionally takes place at a pulp consistency of 10-12%. As alkali source white liquor, which is an internally generated alkali, is most frequently utilised. However, since it is experienced, in connection with the introduction of the oxygen delignification technique in the 1970""s, that untreated white liquor resulted in negative effects to the quality of the pulp, the white liquor is today oxidised in such a way that the sulphide is converted to thio-sulphate. The oxidising may be performed by means of air or pure oxygen gas. The filtrate of the oxygen delignification stage is normally also recycled to the chemical recovering cycle.
After the oxygen delignification the cellulose pulp is bleached in order to remove the last residues of lignin and thus a light and pure pulp is obtained. In conventional bleaching, chlorine dioxide and peroxide are included, but also completely chlorine-free alternatives with ozone or peracetic acid are used commercially today. Old, but still used bleaching agents, are chlorine and hypochlorite. The negative effects of the bleaching filtrates, if they are released, has resulted in, partly, a reduction of the bleaching need, partly a specific reduction of the chlorine chemical use and partly attempts to recycle or recover the bleaching filtrates. One way of reducing the bleaching need is to perform more delignification work in the oxygen delignification stage.
However, there are an upper limit for how much oxygen gas which may be supplied to the delignification stage and this restricts the possibilities to increase the delignification work, since it is not possible to mix too large gas quantities into the mixing devices which are available today. If one increases the gas quantity or gas volume above determined limits channels are formed in the pulp and at least a part of the supplied and expensive oxygen gas will pass through the pulp without effecting any delignification work. One way of solving this problem is to utilise more than one mixing device, which of course involves higher costs.
EP-A-543 135 describes a method for producing oxidised white liquor and totally oxidised white liquor. In this patent application different tries are described, in which oxidised white liquor is utilised as an alkali source in different process steps during the pulp production. EP-A-543 135 includes a brief statement that fully oxidised white liquor may be preferred over partially oxidised liquor for the oxygen delignification of pulps from certain types of woods without giving any teaching as how to such delignification is to be performed or to which types of woods such a delignification would be applicable.
EP-A-792 395 describes a method for regenerating the rest gas from an ozone bleaching stage and utilise this rest gas in the oxygen delignification.
The object of the present invention is to improve the efficiency of the oxygen delignification.
This object is obtained by a method for an integrated treatment of cellulose pulp, including at least the following steps of:
providing said cellulose pulp;
providing a determined white liquor including alkali and sulphur components;
providing an oxygen-containing gas;
oxidising the sulphur components of the white liquor by the supply of a part of said gas in such a way that at least a substantial part of the sulphur is present in the form of sulphate;
transporting the cellulose pulp having a certain kappa number to at least one mixing device;
supplying oxidised white liquor from said oxidising step to the cellulose pulp;
supplying a part of said gas to the cellulose pulp in said mixing device;
mixing of the cellulose pulp with the oxidised white liquor and said gas in said mixing device and
transporting the cellulose pulp from said mixing device to a delignification reactor for oxygen delignification of the cellulose pulp, wherein the kappa number is reduced.
By utilising oxidised white liquor including sulphate in the oxygen delignification stage instead of partially oxidised white liquor including thio-sulphate a plurality of advantages are obtained. In such a way one may avoid that a part of the oxygen supplied to be oxygen delignification reactor is used to oxidise the sulphur components (mainly thio-sulphate) and instead is utilised to increase the delignification work. This involves only a displacement of the addition of oxygen. No additional oxygen is consumed for the oxidation of the sulphur components when this now is done in the preceding oxidising of the white liquor, i.e. outside the delignification reactor. The method according to the invention enables the achievement of many advantages. For instance, it is possible, if one starts with a relatively high lignin content in the pulp into the oxygen delignification stage to increase the wood yield, which involves direct economical advantages and indirect advantages since the capacity in the pulper increases and the load on the chemical recovery decreases. If one decreases the lignin content from the delignification stage the successive bleaching need is reduced, which is advantageous both from an environmental and economical point of view.
A further advantage is that the heat from the exothermic oxidation of the sulphur components is developed outside the oxygen delignification reactor. This enables a better control of the temperature within the delignification reactor during the delignification. Consequently, the cellulose pulp may, according to an embodiment of the invention, be heated before it is supplied to the delignification reactor for maintaining a determined and substantially uniform temperature level during the delignification. The temperature of the totally oxidised white liquor, when it is mixed into the pulp, may thus be chosen with regard to an optimal delignification. Preferably, said temperature level is between 70xc2x0 C. and 120xc2x0 C., for instance 85xc2x0 C. and 100xc2x0 C. Advantageously, the temperature of the oxidised white liquor is measured and the oxidised white liquor, which is supplied to the cellulose pulp, is cooled in response to this measurement. In such a way, the heat formed during the oxidising of the sulphur components may be absorbed and utilised for preheating the cellulose pulp. It is also to be noted that the method according to the invention also enables the achievement of a uniform temperature distribution in the delignification reactor, i.e. one may avoid local temperature peaks in the cellulose pulp, which previously could include local temperature increases of 70xc2x0 C. and thus have a negative influence on the selectivity.
According to a further embodiment of the invention, the method includes controlling the quantity of oxidised liquor supplied to the cellulose pulp in such a way that a desired, determined alkali content profile is maintained during the deliginiflcation step. When, according to the prior art, the oxidation of the sulphur components takes place within the oxygen delignification reactor, the alkali content has to be extra high initially in order to compensate for the alkali consumption of the sulphur oxidation. A too high alkali content is negative to the selectivity since it does not only result in removal of lignin but also in a decomposition of cellulose fibres. By oxidising the sulphur components outside the delignification reactor a greater freedom in the design of the alkali content profile (i.e. the alkali content in the pulp as a function of the time) in the delignification step so that a desired selectivity may be maintained and thus the quality of the pulp produced may be raised. Consequently no increased quantity of alkali is consumed in order to achieve the same delignification result as according to the prior art. Possibly, the total alkali need may be reduced since one may avoid the alkali consuming cellulose/hemicellulose reactions in the delignification step. Advantageously, a parameter, which is related to the content of alkali of the cellulose pulp, for instance the pH-value, is measured at at least one position in the delignification step, wherein said control is performed in response to said measurement.
According to a further embodiment of the invention, the delignified cellulose pulp is supplied to a bleaching process. Reducing as well as oxidising bleaching is possible. For instance, the pulp may be bleached by ozone, peroxide, chlorine dioxide, peracetic acid etc. Advantageously, the bleaching process includes peroxide and/or peroxide/oxygen bleaching steps, wherein oxidised white liquor from said oxidising step is supplied to the cellulose pulp in the bleaching process. According to the prior art, pure NaOH is normally employed as alkali during the peroxide bleaching. By replacing NaOH by oxidised white liquor, the need of externally added alkali may be reduced.
According to a further embodiment of the invention, the cellulose pulp has a pulp consistency which is at least 5% and at most 20%, preferably at least 8% and at most 15% and especially at least 10% and at most 12%, for instance 11%. The cellulose pulp may include chemical pulps produced in digesting processes such as for instance sulphate pulping, sulphate pulping, sulphite pulping with polysulphide, ASAM, MILOX.
According to a further embodiment of the invention said oxygen-containing gas may include at least 60%, at least 70%, at least 80% or at least 90% oxygen. Due to the higher efficiency in the oxygen delignification it is thus possible to utilise less pure oxygen than previously. Of course the best result is obtained by a substantially pure oxygen, which in a commercial context may include about 90-95% or 99-100% oxygen. Very good results also ought to be obtained by a gas which includes 86-88% oxygen.
According to a further embodiment of the invention, said part of the oxygen-containing gas supplied to the mixing device includes a determined quantity of O2 in relation to the reduction of the kappa number and ton of pulp. Thereby, said determined quantity is at the most 1, 7 kg, preferably at the most 1, 5 kg, more preferably at the most 1, 3 kg, and most preferably at the most 1, 2 kg O2 per reduced kappa number and ton of pulp.
According to a further embodiment of the invention the pulp substantially consists of softwood pulp. According to another embodiment, the pulp substantially consists of hardwood pulp.
According to a further embodiment of the invention, the kappa number, during said oxygen delignification step, is reduced by at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, or most specifically at least 75%.